Extreme low-k dielectric film scheme for advanced interconnect

ABSTRACT

An extreme low-k (ELK) dielectric film scheme for advanced interconnects includes an upper ELK dielectric layer and a lower ELK dielectric with different refractive indexes. The refractive index of the upper ELK dielectric layer is greater than the refractive index of the lower ELK dielectric layer.

TECHNICAL FIELD

The present invention relates to the formation of dielectric layersduring fabrication of integrated circuits on semiconductor wafers, andparticularly to the formation of extreme low-k dielectric films foradvanced interconnects.

BACKGROUND

As the density of semiconductor devices increases, however, theresistance capacitance (RC) delay time increasingly dominates thecircuit performance. To reduce the RC delay, there is a desire to switchfrom conventional dielectrics to low-k dielectrics, which have adielectric constant less than SiO₂ or about 4 to prevent cross-talkbetween the different levels of metalization and to reduce device powerconsumption. Low-k dielectrics may also include a class of low-kdielectrics frequently called extreme low-k (ELK) dielectrics, whichhave a dielectric constant less than about 2.5. One of current ELKmaterials is a porous low-k material, which is particularly useful asinter-metal dielectrics (IMDs) and as interlayer dielectrics (ILDs) forsub-micron technology, or even for 65 nm node or 45 nm node or beyondtechnology. The porous low-k dielectric materials produced by spin-onand chemical vapor deposition processes or by a self-assembly processtypically require a curing process subsequent to the deposition. Insteadof thermally curing or plasma treating, the porous low-k dielectrics canbe UV cured at substantially shorter times or at lower temperatures toeliminate the need for prior furnace curing and therefore reducing thetotal thermal budget, while maintaining or reducing the dielectricconstant. However, during the UV curing process, the porous low-kdielectric layer (i.e., a porogen doped SiCO film) only absorbs about40% UV light, while 60% UV light passes through underlying layers. Thiscauses a decrease in UV curing efficiency that needs longer cure timeand lower WPH. The UV penetration issue also degrades the film adhesionof the under layers (i.e., the adhesion between an etch-stop layer and acopper interconnect) that may requires an additional curing process onthe cured ELK dielectric layer and front-end of the line (FEOL) devices.

There is therefore a need in the integrated circuit manufacturing art todevelop a manufacturing process whereby porous low-k dielectric layersmay be formed to improve UV curing efficiency and eliminate the UVpenetration issue.

SUMMARY OF THE INVENTION

Embodiments of the present invention include an extreme low-k dielectricfilm applied to inter-metal dielectric layers for advancedinterconnects. The extreme low-k dielectric film includes dual layerswith different refractive indexes measured at the same UV lightwavelength for prevent a UV light penetrating to under layers during asubsequent UV curing process, enhancing UV curing efficiency and savingUV light.

In one aspect, the present invention provides a semiconductor deviceincluding a first extreme low-k (ELK) dielectric layer formed over asemiconductor substrate, and a second ELK dielectric layer formedbetween the semiconductor substrate and the first ELK dielectric layer.The first ELK dielectric layer has a first refractive index for a UVlight at a predetermined wavelength. The second ELK dielectric layer hasa second refractive index for a UV light at the predeterminedwavelength. The first refractive index is greater than the secondrefractive index.

In another aspect, the present invention provides a semiconductor deviceincluding a semiconductor substrate with a conductive region formedtherein, an etch stop layer formed on the semiconductor substrate, afirst ELK dielectric layer formed over the etch stop layer, a second ELKdielectric layer formed between the etch stop layer and the first ELKdielectric layer, and a dual damascene structure formed in the ELKdielectric layers and electrically connected with the conductive region.The first ELK dielectric layer has a first refractive index for a UVlight at a predetermined wavelength. The second ELK dielectric layer hasa second refractive index for a UV light at the predeterminedwavelength. The first refractive index is greater than the secondrefractive index.

In another aspect, the present invention provides a semiconductor deviceincluding a semiconductor substrate with a conductive region formedtherein, an etch stop layer formed on the semiconductor substrate, anELK dielectric layer formed over the etch stop layer, an air gap formedbetween the etch stop layer and the ELK dielectric layer, and a dualdamascene structure formed in the ELK dielectric layer and the air gapto be electrically connected with the conductive region. The ELKdielectric layer has a refractive index greater than 1.0.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects, features and advantages of this inventionwill become apparent by referring to the following detailed descriptionof the preferred embodiments with reference to the accompanyingdrawings, wherein:

FIGS. 1˜3 are cross-sectional views of a portion of a multi-levelsemiconductor device at stages in an integrated circuit manufacturingprocess.

FIG. 4 is a cross-sectional diagram illustrating an exemplary embodimentof an extreme ELK dielectric film for advanced interconnects.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Preferred embodiments of the present invention provide a novel scheme ofextreme low-k dielectric (ELK) films used as IMD layers or ILD layers inback-end of the line (BEOL) interconnects or front-end of the line(FEOL) interconnects for sub-micron technology (i.e., 65 nm and 45 nmand 32 nm node or beyond technology). As used throughout thisdisclosure, the term “extreme low-k (ELK)” means a dielectric constantof 2.5 or less, including the term “porous low-k” referring to adielectric constant of a dielectric material of 2.0 or less. The ELKdielectric films are advantageously used with silicon oxide based low-kdielectric materials having an interconnecting porous structure and adielectric constant of less than about 2.5.

Reference will now be made in detail to the present embodiments,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers are used in the drawings and thedescription to refer to the same or like parts. In the drawings, theshape and thickness of one embodiment may be exaggerated for clarity andconvenience. This description will be directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art. Further, when a layer is referred toas being on another layer or “on” a substrate, it may be directly on theother layer or on the substrate, or intervening layers may also bepresent.

In an exemplary embodiment, FIGS. 1˜3 show cross-sectional views of aportion of a multi-level semiconductor device at stages in an integratedcircuit manufacturing process.

Referring to FIG. 1, a conductive region 12 is formed in a semiconductorsubstrate 10 by conventional processes known in the micro-electronicintegrated circuit manufacturing art followed by deposition of anetching stop layer 14 overlying the semiconductor substrate 10. Then, anELK dielectric film scheme 20 including an upper ELK dielectric film 18and a lower ELK dielectric film 16 with different refractive indexes isdeposited on the etch stop layer 14. The refractive index is a constantfor a given transparent material. Different wavelengths are refracteddifferent amounts in a given material so it usually has different valuesfor different wavelengths of light. The term “different refractiveindexes” is meant two refractive indexes measured for a UV light at apredetermined wavelength.

The semiconductor substrate 10 is a substrate as employed in asemiconductor integrated circuit fabrication, and integrated circuitsmay be formed therein and/or thereupon. The term “semiconductorsubstrate” is defined to mean any construction comprising semiconductormaterial, for example, a silicon substrate with or without an epitaxiallayer, a silicon-on-insulator substrate containing a buried insulatorlayer, or a substrate with a silicon germanium layer. The term“integrated circuits” as used herein refers to electronic circuitshaving multiple individual circuit elements, such as transistors,diodes, resistors, capacitors, inductors, and other active and passivesemiconductor devices. The conductive region 12 is a portion ofconductive routes and has exposed surfaces that may be treated by aplanarization process, such as chemical mechanical polishing. Suitablematerials for the conductive regions may include, but not limited to,for example copper, aluminum, copper alloy, or other mobile conductivematerials. Copper interconnect level may be the first or any subsequentmetal interconnect level of the semiconductor device. The etch stoplayer 14 for controlling the end point during subsequent etchingprocesses is deposited on the above-described semiconductor substrate10. For example, the etch stop layer 24 is silicon nitride (e.g., SiN,Si₃N₄) or silicon carbide (e.g., SiC) formed by a conventional CVD,LPCVD, PECVD, or HDP-CVD process.

The upper ELK dielectric film 18 is preferably formed with an index ofrefraction (n₁) greater than an index of refraction (n₂) of the lowerELK dielectric film 16. In an embodiment, n₁ is a value greater than orequal to 1.35 for a UV light at a wavelength of 600-700 nm, morepreferably at a wavelength of about 677 nm. For example, the upper ELKdielectric film 18 is a silicon oxide based low-k material layer havinga porous structure, which can be adapted to a porogen-doped SiCO-basedfilm formed by incorporating a pore generating material (a porogen) intoa carbon-doped oxide using plasma CVD such as PECVD including RPCVD orthermal CVD. The upper ELK dielectric film 18 is preferably deposited toa thickness of about 50 Angstroms to about 2000 Angstroms, for example,although it may comprise other thicknesses. One skilled in the art willrecognize that the preferred thickness range will be a matter of designchoice and will likely decrease as device critical dimensions shrink andprocessing controls improve over time.

The lower ELK dielectric film 16 is preferably formed with an index ofrefraction (n₂) lower than an index of refraction (n₁) of the upper ELKdielectric film 18. In an embodiment, n₂ is a value between about 1.0and about 1.35 for a UV light at a wavelength of 600-700 nm, morepreferably at a wavelength of about 677 nm. For example, the lower ELKdielectric film 16 with an index of refraction (n₂) of 1.0˜1.35 is asilicon oxide based low-k material layer having a porous structure,which can be adapted to a SiCO-based film formed by plasma CVD such asPECVD (plasma enhanced CVD) including RPCVD (remote plasma CVD) orthermal CVD. The lower ELK dielectric layer 16 and the upper ELKdielectric layer 18 may be deposited in-situ or ex-situ. In anotherembodiment, the lower ELK dielectric film 16 with an index of refraction(n₂) of about 1.0 is an air gap formed by thermal decomposition, forexample depositing thermally degradable polymer as a sacrificialmaterial and performing a UV curing after a post CMP stage on acompleted interconnect structure embedded in the scheme 20. Thus, thelower ELK dielectric layer 16 and the upper ELK dielectric layer 18 areformed ex-situ. The lower ELK dielectric film 16 is preferably depositedto a thickness of about 30 Angstroms to about 2500 Angstroms, forexample, although it may comprise other thicknesses. One skilled in theart will recognize that the preferred thickness range will be a matterof design choice and will likely decrease as device critical dimensionsshrink and processing controls improve over time.

As the ELK dielectric film scheme 20 including the dual ELK dielectriclayers 16 and 18 with different refractive indexes n₁ and n₂ iscompleted, a UV curing process is performed in a chamber. Referring toFIG. 2, an exemplary reflector 22 is provided in the chamber so that theemitted UV light is appropriately reflected and angles of the reflector20 are adjustable so as to be able to uniform the illumination. Thedirection of the refraction for the UV light passing through the upperELK dielectric film 18 is primarily determined by Snell's law. Snell'slaw holds that n₁ sin θ₁=n₂ sin θ₂, wherein n₁ is the refractive indexof the upper ELK dielectric film 18, n₂ is the refractive index of thelower ELK dielectric film 16, θ₁ is the angle that the UV light in theupper ELK dielectric film 18 makes at the refractive interface withrespect to a normal reference line, θ₂ is the angle that the UV light inthe lower ELK dielectric film 16 makes at the refractive interface withrespect to the normal reference line. If the angle of incidence (i.e.θ₁) is greater than or equal to a critical angle (i.e. θ_(C)), then theUV light undergoes total internal reflection within the upper ELKdielectric film 18. Total internal reflection occurs in accordance withθ_(c)=sin⁻¹ (n₂/n₁) for n₁>n₂. Thus, in the embodiment, when thereflector 22 is adjusted to make θ₁ greater than or equal to θ_(C), theUV light does not enter the lower ELK dielectric film 16, but isreflected internally in the upper ELK dielectric film 18.Experimentally, compared with the conventional single ELK dielectricfilm scheme, the dual ELK dielectric scheme with dual refractive indexesof the invention can prevent the UV light penetrating into under layersso as to save about 60% UV light, which advantageously improves UVcuring efficiency.

An exemplary dual damascene structure 24 formed in the ELK dielectricfilm scheme 20 is shown in FIG. 3. One or more hardmask/bottomanti-reflectance coating (BARC) layers may be provided over the ELKdielectric film scheme 20 at an appropriate thickness, to minimize lightreflectance in a subsequent photolithographic patterning process.Lithographic and etching processes are then carried to form a dualdamascene opening including for example an upper via opening and a lowertrench opening. A barrier layer, preferably including one of arefractory metal, refractory metal nitride, and silicided refractorymetal nitride layer, for example Ta, Ti, W, TaN, TiN, WN, TaSiN, TiSiN,and WSiN is deposited to line the dual damascene opening. For fillingthe dual damascene opening, copper deposition processes, for exampleelectrochemical deposition preceded by deposition of a copper seed layeris carried out, and then a copper ECD process is performed followed by aCMP process to remove the excess portion of copper layer, barrier layer,and at least a portion of hardmask/BARC layer to complete the formationof the dual damascene structure 24.

Although the present invention is explained by reference to an exemplaryELK dielectric film scheme 20 on the etch stop layer 14 with arefractive index of about 2.0, it will be appreciated that the ELKdielectric film scheme 20 of the present invention applies generally toa dielectric layer with a refractive index n₃ greater than n₂ of thelower ELK dielectric layer 16. FIG. 4 shows another embodiment providingthe ELK dielectric film scheme 20 on a TEOS oxide layer 26 for anintegrated circuit manufacturing process. The TEOS oxide layer 25 has arefractive index of about 1.46.

Although the present invention has been described in its preferredembodiments, it is not intended to limit the invention to the preciseembodiments disclosed herein. Those skilled in this technology can stillmake various alterations and modifications without departing from thescope and spirit of this invention. Therefore, the scope of the presentinvention shall be defined and protected by the following claims andtheir equivalents.

1. A semiconductor device, comprising: a semiconductor substrate; afirst dielectric layer with a dielectric constant not greater than 2.5formed over said semiconductor substrate; and a second dielectric layerwith a dielectric constant not greater than 2.5 formed between saidsemiconductor substrate and said first dielectric layer; wherein saidfirst dielectric layer has a first refractive index for a UV light at apredetermined wavelength, said second dielectric layer has a secondrefractive index for a UV light at said predetermined wavelength, andsaid first refractive index is greater than said second refractiveindex.
 2. The semiconductor device of claim 1, wherein said firstrefractive index is greater than or equal to about 1.35 for a UV lightat a wavelength of 600˜700 nm.
 3. The semiconductor device of claim 1,wherein said second refractive index is in the range from about 1.0 toabout 1.35 for a UV light at a wavelength of 600˜700 nm.
 4. Thesemiconductor device of claim 1, wherein said first dielectric layer isa porous SiCO-based dielectric layer.
 5. The semiconductor device ofclaim 1, wherein said second dielectric layer is a porous SiCO-baseddielectric layer.
 6. The semiconductor device of claim 1, furthercomprising a damascene structure in said first dielectric layer and saidsecond dielectric layer, electrically connecting with a conductiveregion formed in said semiconductor substrate.
 7. The semiconductordevice of claim 1, further comprising an etch stop layer between saidsecond dielectric layer and said semiconductor substrate.
 8. Thesemiconductor device of claim 7, further comprising a TEOS oxide layerbetween said second dielectric layer and said etch stop layer.
 9. Thesemiconductor device of claim 1, wherein said first dielectric layer hasa thickness of about 50 Angstroms to about 2000 Angstroms.
 10. Thesemiconductor device of claim 1, wherein said second dielectric layerhas a thickness of about 30 Angstroms to about 2500 Angstroms.
 11. Asemiconductor device, comprising: a semiconductor substrate comprising aconductive region formed therein; an etch stop layer formed on saidsemiconductor substrate; a first dielectric layer with a dielectricconstant not greater than 2.5 formed over said etch stop layer; a seconddielectric layer with a dielectric constant not greater than 2.5 formedbetween said etch stop layer and said first dielectric layer; and a dualdamascene structure formed in said first dielectric layer and saidsecond dielectric layer, electrically connected with said conductiveregion of said semiconductor substrate; wherein said first dielectriclayer has a first refractive index for a UV light at a predeterminedwavelength, said second dielectric layer has a second refractive indexfor a UV light at said predetermined wavelength, and said firstrefractive index is greater than said second refractive index.
 12. Thesemiconductor device of claim 11, wherein said first dielectric layer isa porous SiCO-based dielectric layer with a refractive index greaterthan or equal to about 1.35 for a UV light at a wavelength of 600˜700nm.
 13. The semiconductor device of claim 11, wherein said seconddielectric layer is a porous SiCO-based dielectric layer with arefractive index in the range from about 1.0 to about 1.35 for a UVlight at a wavelength of 600˜700 nm.
 14. The semiconductor device ofclaim 11, further comprising a TEOS oxide layer between said seconddielectric layer and said etch stop layer.
 15. The semiconductor deviceof claim 11, wherein said first dielectric layer has a thickness ofabout 50 Angstroms to about 2000 Angstroms.
 16. The semiconductor deviceof claim 11, wherein said second dielectric layer has a thickness ofabout 30 Angstroms to about 2500 Angstroms.
 17. A semiconductor device,comprising: a semiconductor substrate comprising a conductive regionformed therein; an etch stop layer formed on said semiconductorsubstrate; a dielectric layer with a dielectric constant not greaterthan 2.5 formed over said etch stop layer; an air gap formed betweensaid etch stop layer and said dielectric layer; and a dual damascenestructure formed in said dielectric layer and said air gap, electricallyconnected with said conductive region of said semiconductor substrate;wherein said dielectric layer has a refractive index greater than 1.0.18. The semiconductor device of claim 17, wherein said first dielectriclayer is a porous SiCO-based dielectric layer with a refractive indexgreater than or equal to about 1.35 for a UV light at a wavelength of600˜700 nm.
 19. The semiconductor device of claim 17, further comprisinga TEOS oxide layer between said air gap and said etch stop layer. 20.The semiconductor device of claim 17, wherein said dielectric layer hasa thickness of about 50 Angstroms to about 2000 Angstroms.